Kitchen Extension with Open-Plan Living — Design, Costs, and What to Expect

• **Extension depth and width**:
• The size of the extension determines the character of the new space:
• •**3m deep**: a meaningful addition — creates a usable kitchen area with good natural light from rooflights and rear glazing. The existing kitchen/dining room becomes the living space.
• •**4m deep**: the most popular depth — creates a fully open-plan kitchen-diner with a distinct island or breakfast bar and enough space for dining. Comfortable family space.
• •**5–6m deep**: a generous space — can accommodate a kitchen, dining table for 8–10, and a separate seating area all within the extension. Often requires planning permission (exceeds PD limits).

**Width**: most Victorian mid-terraces are 4–5m wide internally. The rear extension typically matches this width or takes in the width of the rear outrigger if the house has one. Side return extensions widen the ground floor by 1–2m, which is particularly valuable in narrow terraces.

**Roof form and natural light**: For extensions over 3m deep, natural light from the rear glazing alone is insufficient to light the interior. Rooflights (flat rooflights in the flat-roof extension, or pitched rooflights in a lean-to) are essential. A common design: full-width sliding or bifold doors at the rear, plus 2–4 rooflights in the flat roof above the kitchen.

**The structural wall removal**: The rear wall of the existing house, and often a chimney breast or load-bearing partition within the ground floor, will be removed to open the new extension into the existing house. This is typically the most technically demanding element — requiring structural engineering and one or more steel beams.

**Island vs no island**: For extensions 4m or deeper, a kitchen island is almost universally included in the brief. It provides additional worktop and storage, creates a natural visual divide between cooking and dining zones, and often incorporates seating for casual meals. Allow at least 900mm circulation width on all sides of the island.

• **Demolition and enabling works**:
• •Demolition of the existing rear extension or utility room (most Victorian houses have a smaller rear outrigger that is demolished to clear the footprint)
• •Removal of the rear external wall (or most of it, with the structural opening formed at this stage)
• •Disconnection of existing services in the demolition zone

• **Foundations and structure**:
• •New foundations for the extension (typically strip or trench-fill concrete foundations, 600–900mm deep for a single-storey rear extension)
• •Blockwork walls to full height
• •RSJ beam(s) at the structural opening(s) between new extension and existing house
• •Flat roof structure (timber joists or steel deck)

• **Waterproof shell**:
• •EPDM or GRP flat roof membrane
• •Bifold or sliding door installation
• •Rooflights installation
• •External render or brickwork to extension walls

• **First fix**:
• •Extension plumbing: kitchen sink, dishwasher, washing machine positions; underfloor heating pipes if specified
• •Extension electrics: sockets, lighting circuits, extractor, appliance circuits
• •Underfloor heating manifold if specified (liquid screed typically follows first fix and precedes plastering)

• **Plastering**:
• •Internal plaster skim to all surfaces
• •Floor screed if underfloor heating is specified

• **Second fix and fit-out**:
• •Kitchen installation (units, worktops, appliances)
• •Underfloor heating commissioning (if fitted)
• •Second fix electrics and lighting
• •Decoration
• •Flooring installation (large-format tiles, engineered wood, or polished concrete are all popular in open-plan extensions)

**Programme**: a 4m x 4.5m single-storey rear extension creating an open-plan kitchen-diner takes approximately 12–16 weeks on site from start to practical completion.

• Open-plan kitchen extension costs depend on:
• •Extension size (depth, width)
• •Kitchen quality and appliance specification
• •Floor finish (tile, wood, polished concrete)
• •Glazing specification (standard vs slim-frame bifold or sliding)
• •Whether underfloor heating is included
• •Level of internal reconfiguration

**Approximate costs (London, 2025) — build only (excluding architect fees)**:

**3m deep single-storey rear extension + kitchen installation (mid-spec)**: £55,000–£80,000 (Includes: all trades, structural opening, bifold doors, 2 rooflights, kitchen unit supply and installation, appliances, flooring, decoration)

**4m deep extension + kitchen installation (mid-spec)**: £70,000–£100,000

**4m deep extension + kitchen + underfloor heating**: £80,000–£115,000 (Adding underfloor heating: +£6,000–£12,000 for the system, screed, and commissioning)

**5m deep extension + premium kitchen + slim-frame sliding doors**: £100,000–£150,000

**Notes on the kitchen cost element**: Kitchen unit supply is often client-provided (the homeowner sources kitchen units directly from IKEA, John Lewis, Howdens, etc.). The contractor installs the units and connects the appliances. On this basis, the cost above for the kitchen includes installation (£2,000–£6,000 for a standard kitchen fit) but not the units (which can range from £3,000 for IKEA to £30,000+ for bespoke cabinetry).